Well make sure it has a dedicated graphics card and most importantly not an Intel HD video solution. SU is an OpenGL based program and needs a good video card. What is your budget is the real question and what OS do you want, Windows7 or OSX? I see AutoCAD 2008 so I will assume that you will go the Windows route, correct? Do you want a small screen or a larger one?
Here is a good Asus for $730 - http://www.newegg.com/Product/Produc...82E16834230236
Intel Core i5 2450
1GB nvidia GF GT610M graphics